(a) Field of the Invention
The present invention relates to an improved fluid operated stepping motor having accurate resolution and a high output torque.
(B) Description of Prior Art
The use of stepping motors in the fields of registering, indexing and positioning is usually preferred to continuous-running motors because of absence of feedback loops as in conventional servo systems.
Stepping motors can be classified into the following two categories according to their output characteristics:
Low torque, high maximum stepping speed, as in most electric steppers (such as Sigma, model 20-22-150D200 - F 1.5, having a maximum speed of 10,000 steps/sec and a very low torque at that speed, and a torque of 30 oz-in. at 50 steps/sec;
HIGH TORQUE FAIR STEPPING SPEED. Commercially available ones that fall under this category are mainly electrohydraulic (typical characteristics are 2000 steps/sec and a torque figure around 87 lb-in. at 1000 PSI-G oil pressure).
The cost of the necessary accessories as pulse generating and translator circuits, hydraulic supply package and so forth, in addition to that of the motor itself, makes them unsuitable for certain class of industrial applications.
For a range of medium, torques and low stepping speeds, there has been several developments of pneumatic steppers. For instance, there is known a motor which resembles a Wankel engine, having a triangular rotor. Precision machining is therefore mandatory, and resolution is of the order of several radians. Discrepancy between actual and theoretical resolutions is appreciable. A second motor is also known in which an eccentric rotor is forced to rotate by an air pressure differential across two chambers. The accuracy of the resolution of such motor is bad and the torque output is very small. Another known motor is provided having a stepping speed of up to 250 steps/sec with good torque-speed characteristics. It is similar to a multiphase electric stepper, in having a flexible spline as a rotor, which is actuated by a number of elastic bags pressurized sequentially by a fluidic translator circuit. The major disadvantage of this motor lies in the delicate construction of the spline-rotor.
A further type of fluid operated stepping motor is known as described in U.S. Pat. No. 3,661,059 issued on May 9, 1972 to Chandler Evans Inc. This patent teaches a complex stepping motor design which comprises a plurality of pistons disposed parallel to one another on an axis which is parallel to the axis of rotation. This type of motor also requires a complex logic circuit and utilizes a symmetric cam. The control circuit is necessary for the operation of the motor to identify the sequential operation of the cylinders. Also, the motor incorporates valving in the motor structure which makes it bulky and unlikely not capable of being miniaturized. Also, the symmetrical cam is provided with teeth having more than two surfaces. Thus, there is the need to provide a fluid operated stepping motor which is relatively easy to construct, and which utilizes a more simple principal whilst providing excellent accuracy in resolution and a high output torque.